International Journal of Geomagnetism and AeronomyVol. 4, No. 2, August 2003

Slow dynamics of
photospheric regions
of
the open
magnetic field of
the Sun, solar activity phenomena, substructure of the
interplanetary medium and near-Earth disturbances of the early 23rd cycle:
March-June 1997 events

Abstract

Slow (from one rotation to another)
dynamics of the photospheric regions of the open magnetic field of
the Sun (ORs), solar activity phenomena (coronal holes, active
filaments, flares), subsector structure of the coronal field on
the source surface, and near-Earth interplanetary medium are
considered at the piece of the solar surface ~120
o wide
centered at the CL
150o Carrington latitude in the sequence
of the CR 1920-CR 1923 Carrington rotations (March-June 1997). A
special attention is paid to a prominent event of this period on
12-18 May 1997. It is demonstrated that in the coarse of the slow
dynamics of ORs on the part of the Sun chosen, there was formed in
the CR 1922 a specific configuration of ORs on the photosphere
which generated a complicated structure of the magnetic field at
the source surface and in the interplanetary field with a strong
bow of the heliospheric current sheet (HCS) and a "joint" between
HCS and the intersector boundary. A complicated solar activity
complex was formed near this "joint", the complex consisting of a
flare active region (AR), low-latitude coronal hole (CH), and
active filament (ADF). During CR 1920-CR 1922, there was observed
a dynamical high-velocity flow which generated in CR 1921-CR 1922
a near-Earth MHD disturbance with three-phase temporal dynamics
typical for the disturbances near HCS. The destabilization of the
AR-CH-ADF-HCS complex in CR 1922, occurrence of a coronal mass
ejection (CME) in AR, and the interaction between CME, HCS, ADF,
and CH led to a significant modification of the recurrent
high-velocity flow (appearance of the main shock wave and magnetic
cloud, multiple crossings of HCS, velocity increase, and a
"loading" of the flow by the active filament substance) and to a
sharp increase of its geoefficiency. Possible scenarios of the
12-18 May 1997 events are discussed: a simple one with the
magnetic cloud from AR 8038 [Webb et al.,2000a]
and a complicated one which includes a formation of
the cloud due to the interactions in the activity complex with an
HCS tiring-instability
[Kivelson and Khurana, 1995]
and its
modeling by the heliospheric electrojet field
[Ivanov and Romashets, 1999].

1. Introduction

There are arguments in the solar-terrestrial physics in favor of
the statement that many interplanetary MHD disturbances are
generated by complicated solar sources, the latter being various
combinations of coronal holes, filaments, flares, and active
regions
[Bravo et al., 1998;
Burlaga et al., 1987;
Crooker and McAlister, 1997;
Dryer and Smith, 1987;
Gonzalez et al., 1996;
Gosling, 1993;
Ivanov, 1996, 1998;
Kahler, 1991;
Mogilevsky et al., 1997].
In the complicated source concept, very perspective in
our opinion is its representation as a large unstable configuration
of the solar magnetoplasma in which coronal holes, filaments, and
active regions are closely interrelated and are only specific
components of this source (complex), characterizing its formation,
destabilization, and decay
[Gosling, 1993;
Mogilevsky et al.,1997].

There are two fundamental problems concerning the phenomenology
of such complexes: first, study of their dynamics at characteristic
times of the order of a few solar rotations, and second, studies of the
interactions between its components, especially at the source
destabilization phase leading to complicated interplanetary
disturbances.

It has been shown recently that the activity complexes appear
(disappear) in mutual collisions (reflections) of photospheric
regions of the open magnetic field of the Sun
[Ivanov and Kharshiladze, 2002;
Ivanov et al., 2001b].
As a result, we propose
to use the dynamics of open regions as a convenient instrument for
determination in advance of the tendencies leading to formation and
destabilization of activity complexes including such determinations
into the analysis of some prominent events of the solar-terrestrial
physics. The analysis of the well-known disturbances of
5-11
January
1997
[Ivanov et al., 2001a;
Ivanov et al., Slow dynamics of photospheric regions of open
solar magnetic field, solar activity, substructure of the interplanetary space
and near-Earth disturbances of the early 23rd solar cycle:
1. December 1996 to February 1997 events,
submitted to
International Journal of Geomagnetism and Aeronomy, 2002,
(hereinafter referred to as
Ivanov et al., submitted manuscript, 2002)]
is an example of such an
approach.
The above papers
demonstrated
that there
was observed a convergence of open photospheric regions at least
one rotation prior to the activity complex destabilization, that
the filament-active region and coronal hole were the most active
components of this complex, and that taking into account of the
interaction between them is important for explanation of the main
characteristics and some peculiarities in the configuration,
structure and dynamics of the interplanetary disturbance observed
near the Earth. At the same time, the set of these characteristics
and features is an experimental requirement which makes it possible
to indicate perspective approaches to the MHD modeling of this
disturbance.

Similar approach is used below to consider in detail the 12-18 May
1997 event. This event is interesting in particular due to the fact
that in this event for the first time the Morton wave was
registered in the extreme ultraviolet
[Thompson et al., 1998],
a
coronal halo was observed
[Plunkett et al., 1998;
Sheeley et al.,1999;
Webb et al., 2000a],
a pair of transient coronal holes
(dimmings) located symmetrically relative the neutral line of the
active region magnetic field was formed
[Hudson et al., 1998;
Thompson et al., 1998],
preflare sigmoid shape of the coronal arcs
was observed
[Glover et al., 2000;
Hudson et al., 1998],
the
photospheric bases of the post-eruptive arc coincided almost
exactly with the magnetic spots of the opposite polarity
[Thompson et al., 1998;
Webb et al., 2000a],
small rapid (during the flare)
and considerable slow (of about a day) variations of the magnetic
flux in the active region were registered
[Lara et al., 2000],
and
the most intense magnetic storm of 1997 was observed
[Bruckner et al., 1998;
Ivanov and Romashets,1998].
Considering the data of the
Wind/Waves radiophysical experiment,
Gopalswamy et al. [[2000]
referred to this event as to a radio-rich event because there
presented in it not only the meter and kilometer, but also
type II
decameter and hectometer radiobursts.

It is desirable to continue the studies of this event in the
following directions:

First, on analogy with the 5-11 January 1997 event
[Ivanov et al.,2001a;
Ivanov et al., submitted manuscript, 2002]
the slow (from one rotation to
another) dynamics of the photospheric regions of the open field,
solar activity complexes and interplanetary medium structure
should be taken into account.

Second, pay attention to the fact that almost in all papers
cited above the interpretation and modeling of the near-Earth
disturbance is based on the consideration of only those solar
events which had been formed in AR 8038 after the 1n(C.1.3) flare
near the central meridian. The fact that these events could have
been altered in interactions in the activity complex was never
taken into account. Relating to this, we would note that the event
have occurred in the vicinity of the heliospheric current sheet
(HCS)
[Ivanov and Romashets, 1999;
Sanderson et al., 1998]
and that
between the active region and HCS an active ( L = 12o ) and dense
( I = 2 ) active filament of the ADF type was observed
(Solar-Geophysical Data, Boulder, NN 637-640, 1998). Therefore, on
the propagation path of the coronal mass ejection (CME) from the
Sun to the Earth an interaction of CME with ADF and HCS should have
occurred. Thus it is interesting to bear in mind that though the
kilometer radiobursts of the type II were observed by the Wind
satellite almost permanently till the arrival of the forward shock
wave to the Earth
(http://lep694.gsfc.nasa.gov/waves),
their intensity was very small
[Reiner et al., 1998],
especially
till about 1800 UT on 13 May. The latter fact makes it possible to
assume that they were screened by the plasma sheet on HCS
[Ivanov and Romashets, 2001].
Thus the presence of the activity complex (AR
+ ADF
+ HCS)
and the above-indicated interactions could be
manifested in the structure and dynamics of the near-Earth
disturbance, in particular, in the appearance of the disturbance
preliminary phase and magnetic cloud closely related to HCS
[Ivanov and Petrov, 1999;
Ivanov and Romashets, 1999;
Ivanov et al., 1999].

Third, pay attention to various approaches to modeling of the
near-Earth magnetic cloud in this disturbance: it is either a field
of the heliospheric electrojet on HCS
[Ivanov and Romashets,1999],
or a field of a powerless cylinder (part of a toroidal
configuration related to the bipolar group in AR)
[Watari et al., 2001;
Webb et al., 2000a].

Fourth, bear in mind that the problem of the plasma origin may be
related to the problem of the magnetic cloud origin. For example,
Webb et al. [2000a]
paid attention to the fact that in this
magnetic cloud neither "plasma density pulse" nor any sign of the
filament substance were observed contrary to the magnetic cloud in
the 10-11 January 1997 event
[Burlaga et al., 1998].
According to
the
Hudson et al. [1998]
estimates based on the dimensions of
"dimmings" in the soft X rays, the cloud contained only a small
part of the total mass of CME. These observations contradict to the
modeling of the initial configuration in which the filament
substance was located within the cloud (magnetic bundle)
[Gibson and Low, 2000].
It is possible that in this event the cloud is
closely related (not only spatially but also by its origin) to HCS
and presents an analog of rarefied plasmoids appearing in the
magnetotail during the reconnection process
[Kivelson and Khurana,1995].
In favor of this assumption probably is the fact that there
was no two-directional electron fluxes in the cloud
[Shodhan et al., 2000;
Webb et al., 2000a].

Fifth, clarify the reason of the discrepancy between the
directions of the magnetic axes of the bipolar group in AR to the
Sun and of the magnetic cloud near the Earth.
Webb et al. [2000a]
suggested that to the moment of its arrival to the Earth orbit the
cloud has turned around by
40o and shifted southward
by
30o from its initial position
such that the cloud axis became located
below the ecliptic plane. There is no such difficulty in the
alternate model of cloud generation on HCS
[Ivanov and Romashets,1999].

The paper contains
initial data and methods
in section 2,
section 3 is
dedicated to the dynamics of the photospheric open field, subsector
structure of the coronal field and solar activity phenomena,
section 4 is dedicated to the dynamics of the subsector structure of
the interplanetary medium, section 5 covers
the 12-16 May, 1997 events,
and section 6 is a discussion.

2. Initial Data and Methods

Figure 1

Figure 2

To determine the spherical coefficients of the Gauss series in one
of the versions of the solar magnetic field potential model with a
source surface
[Kharshiladze and Ivanov, 1994],
the observations of
the photospheric magnetic field at the Wilcox Solar Observatory
(http://sun.Stanford.edu/)
are used. Then the open (going
away into the interplanetary medium) lines of the magnetic field
are projected from the source surface (a spherical surface with
r = 2.5 R
from the center of the Sun) onto the photosphere using the
Levine et al. [1977]
method. In the process of such modeling, the
computer memorizes mutually-unambiguous relation between a
particular photospheric region of the open field (OR) and the
corresponding domain on the source surface. As a result, it becomes
possible to obtain synoptic maps of the Sun in the Mercator
projection. Combined on these maps are ensembles of photospheric
ORs and the subsector structure of the solar magnetic field with
the sector and intersector boundaries (Figures 1 and 2). Further,
the following data were put onto these maps: the boundaries of the
coronal holes in the FeXIV line observed at the Sacramento Peak
Observatory, magnetic fields of the sunspot groups observed at the
Kitt Peak Observatory, active filaments (Table 1), and flares
(Solar-Geophysical Data, Boulder, NN 637-640, 1998).
Then (1) a
piece of the disk with the width
DL = 150o
centered
at the Carrington longitude
L = 140o where the solar
activity events responsible for the near-Earth disturbance on
14-16
May
1997 were observed; and (2) slow (from one rotation to another)
dynamics of the open regions, solar activity events, and the sector
structure of the coronal field on the source surface were
considered on this piece of the disc during four Carrington
rotations (CR 1920-CR 1923: March-June 1997).

The data on the interplanetary magnetic field (IMF) and solar wind
plasma obtained on board the Wind satellite (the leading scientists
were P. Lepping and K. Ogilvie, http://cdaweb.gsfc.nasa.gov)
obtained in the scope of the International Solar-Terrestrial
Physics Program (ISTP) were used to study the dynamics of the
subsector structure of the interplanetary medium and the
correspondence of this dynamics to solar events. A special
attention has been paid to the
14-16
May
1997 disturbance.

3. Photospheric Open Regions, Solar Activity, and the
Field on the
Source Surface

The structure, configuration, and dynamics of the photospheric open
regions, solar activity events (coronal holes, filaments, and
active regions), and coronal magnetic field (sector and intersector
boundaries) on the source surface in the sequence of four Carrington
rotations CR 1920-CR 1923 are considered below.

The photospheric open field (closed circles) is mainly concentrated
in the polar caps (at the heliolatitudes
|F| 70o ).
The
positive and negative polarity lines are located in the Northern
and Southern
hemispheres, respectively.

However, part of the open lines exits outside the polar caps to
lower latitudes (the groups of larger circles designated as
+2 and
-1,
-2 in the
Northern and Southern
hemispheres, respectively).
Moreover, in the Northern hemisphere there is a low-latitude
photospheric open region designated as
+1.

The distance between ORs of opposite polarity,
r > R, manifests a
remote interaction between these ORs which allows for formation of
low-latitude coronal holes (thin curves) and active filaments (line
segments),as well as for a deformation of the sector boundary on
the source surface (solid curve)
[Ivanov et al., 2001b].

The coronal field on the source surface is mainly formed by the
open lines emerging from the polar caps. Moreover, there are several
subsectors ( +1;
+2;
-1;
-2 ) formed by the open lines going out from
the corresponding open photospheric regions shown on the map by
large black circles. The intersector boundaries on the source
surface (the boundaries of subsectors) are shown by thin lines.

It is worth noting the following: (1) The helioprojection of the
Earth passes through two low-latitude coronal holes formed by the
open lines emerging from the photospheric polar regions (from the
caps and OR
+2 and
-2 ).
Levine et al. [1977]
were the first to draw
attention to such specific low-latitude holes. (2) There are several
active filaments in the vicinity of the sector boundary. (3) There is
no active regions (except one very small region near the joint of
HCS and the
-2 intersector boundary). During this rotation, a very
durable (22-28 March) high-velocity flow was observed near the
Earth with complicated profiles of the velocity
V, concentration
n,
and temperature
T, the profiles indicating to a loading of the flow
by the filament substance.

On 15 April the following changes happened in the ensemble of ORs
on the piece of the solar surface considered: (1) in the Northern
hemisphere, OR
+2 disappeared, a high-latitude region was formed at
the OR
+1 meridian, and the OR
+1 region itself expanded northward
toward OR
+3; (2) in the Southern hemisphere, OR
-2 disappeared and OR
-1 was shifted eastward, its area increasing significantly.

Interactions between ORs of the opposite polarity determined by the
distance between the centers of OR
+1 and OR
-1 ( r > R ) still
remains a remote one. The field on the source surface is still
formed mainly be the open lines emerged form the polar caps.
However, the Earth's helioprojection goes though the subsector
structure of the coronal field formed by the open lines going out
from the OR
+1 and OR
-1 midlatitude parts of the photosphere.

The sector boundary is in a considerable nonequilibrium in the
sense that during the
8-11
April
and 12-17 April periods it is
formed by the oppositely directed open lines of the midlatitude OR
+1 and southern polar cap, and OR
-1 and the Northern cap,
respectively. As a result of this nonequilibrium, the sector
boundary gets bows to the north and south from the equator,
respectively.

The following
should be noted

1. Though the total area of the
low-latitude coronal holes was reduced as compared to CR 1920, a
small coronal hole in the vicinity of the joints of the sector and
subsector boundaries is visually seen. Nevertheless, this coronal
hole is still formed by the open lines going out from the OR
-1 photospheric region located at higher latitude that the this hole.

2. Below the subsector boundary of OR
-1, there appeared a low-latitude
active region 8032 of the new solar activity cycle. On 15 April at
1410 UT, there occurred a solar flare of a moderate (maximum)
optical ball SN(1N)/C1.0 in the point with the coordinates S23 E09
(Solar-Geophysical Data, Boulder, NN 637-640,
1998).
It is
interesting that the position of this flare relative the equator
and central meridian was almost mirror as compared to the 12 May
flare (see section 3.3) responsible for the prominent
near-Earth disturbance on 15 May 1997.

3. There was almost no
filaments in the vicinity of the Earth's helioprojection.

During this rotation near the Earth, as well as in CR 1920, a
high-velocity flow from the coronal hole was observed, the flow
having shorter duration and relatively smooth profiles of
V,
n, and
T.

In the Northern hemisphere,
almost complete
dissipation of OR
+1 happened, whereas OR
+3 was shifted eastward. In the
Southern hemisphere, there
further shifting of OR
-1 eastward and equatorward
happened. The interaction between ORs still is
remote, and the subsector structure on the source surface stays
almost unchangeable. The nonequilibrium of the sector structure is
mainly conserved.

The most significant change of the activity complex as compared
with CR 1921 was an appearance of an active filament and active
region in the vicinity of the joint between the sector and
intersector boundaries in addition to the coronal hole which
existed in the previous rotations. In the active region in the N23 W09
point a flare 1n(C.1.3) occurred at 0445 UT on 12 May 1997 and
initiated the solar-terrestrial disturbance on 12-16 May 1997.
Thus the very destabilization of the (A-AD-CH-HCS) activity
complex formed near the joint of the sector and intersector
boundaries between CD 1921 and CR 1922 became a cause of the
corresponding disturbances on 14-16 May 1997.

Therefore, to create a qualitatively correct scenario and MHD model
of this disturbance, one should take into account the entire
complex of the solar activity and the influence of its components
(coronal holes, filaments and heliospheric current sheet) on
generation and propagation of the shock wave, magnetic cloud, and
solar and galactic cosmic rays, whereas in the majority of the
papers dedicated to the 12-16 May 1997 event, only the sporadic
phenomena related to AR 8038 were taken into
account. The heliospheric current sheet was taken into account by
Sanderson et al. [1998] and
the current sheet and active filament were
taken into account by
Ivanov and Romashets [1999] and
Ivanov and Petrov [1999].
Lario [[2000]
noted that corresponding high-velocity
flow measured by Ulysses on 19 May 1997 was recurrent. Till now
nobody has considered the low-latitude coronal hole (Figure 2a) as
one of the sources of the near-Earth disturbance.

Comparing the CR 1922 and CR 1923 rotations, one can conclude that
in the space between OR
+3 and OR
-1 a decay of the
activity complex responsible for the 12-16 May 1997 disturbance
occurred.
Actually,
the active region with the bipolar group has
disappeared, though on the place of the complex there are still
observed two active filaments and a low-latitude coronal hole.
Their mutual position became less compact and their geometric
characteristics changed. These "remnants" of the complex still are
located near the joint of the sector and intersector boundaries,
but the former has a zero inclination relative the equator contrary
to CR 1922.

It is possible that the decay of this complex is related to two
events:
(1) to the energy loss in the May 1997 sporadic events
and (2) to the decrease of the energy income into the interaction
region of the pair of the OR
+3 and OR
-1 open regions. The latter
suggestion is confirmed by a depletion of the OR
+3 region area as
compared with the two previous rotations.

4. Dynamics of the Near-Earth Interplanetary Medium

Figure 3

Figure 4

Now we consider the corresponding sequence of the states of the
near-Earth interplanetary medium observed in the CR 1920 and CR 1923
rotations on board the Wind satellite (Figures 3, 4, 5, 6, 7, and 8). We would like to
demonstrate that the formation, destabilization, and decay of the activity
complex considered in section 3 are manifested in these disturbances.

The wind velocity is characterized by a well-pronounced dynamics. At
the initial stage of the complex development (CR 1920), there is a
complicated profile formed by a sequence of two flows. At the
formation and destabilization stages (CR 1921-CR 1922), there is a
high-velocity flow almost stable during 2 days in the duration,
intensity and profile shape. At the decay phase (CR 1923), there is
a stationary slow solar wind. It is worth noting that the
V profiles in the CR 1921 and CR 1922 rotations are so similar that
one can come to the conclusion
that there is an
absence of any influence of
the known sporadic events of 12 May 1997
[Hudson et al., 1998;
Plunkett et al.,1998;
Thompson et al., 1998;
Webb et al., 2000a, 2000b].

Evidently, this stability of the velocity profile is due to
the
fact that the dynamics of the solar wind velocity near the Earth
was mainly governed by the dynamics of the low-latitude coronal
holes responsible for this flow. Actually, in the beginning (CR 1920,
Figure 1a)
a sequence of two large holes was observed.
However, the first one is formed by the open lines from the polar
cap photosphere, whereas the second is screened from the Earth by
the heliospheric current sheet. Probably this fact predetermined
the low velocity in the first flow and the complicated profile of
the velocity in the second flow. The maximum velocity in the second
flow was caused by a break of the screening due to the Earth
crossing of the heliospheric current and entering the coronal hole.
During the two following rotations (CR 1921 and CR 1922) the
helioprojection of the Earth passed across the same coronal hole
which was stable in its shape and position and formed by the open
lines emerged from the midlatitude photosphere.

Thus the question arises, how the coronal mass ejection 12
of May
1997 influenced the velocity profile near the Earth? To answer this
question we should pay attention to fine features of the velocity
profile near the Earth in Figure 3.
Ivanov and Romashets [1999]
and
Ivanov and Petrov [1999]
paid attention to the fact that the
near-Earth disturbance of 14-16 May 1997 was characterized by a
three-phase time dynamics. A sequence of development (G), main (M),
and recovery (R) phases and also its manifestation in all
parameters of the solar wind plasma and IMF components are typical
for this dynamics. Figure 3 confirms the presence of the
three-phase dynamics in the velocity profiles observed during CR 1921 and
CR 1922.

One should note a small difference in the May and April (CR 1922)
profiles: (1) The development phase is better pronounced in April
than in May, (2) the main phase starts from the SI flow surface in
April and
Sf forward shock wave in May, (3) the directions of
the
velocity changes are different during the main phase: there is a
decrease in April soon after SI and a durable increase in May after
Sf, and (4) the flow is observed between the sector
and one of the
intersector (HCS and SB) boundaries in April and between two
intersector boundaries in May.

These differences may be interpreted (see also the concentration
and IMF profiles) as effects of a modification of the initial
high-velocity flow observed during the CR 1921 rotation (April) and
interactions in the (AR-CH-ADF-HCS) complex formed after the flow
destabilization in May 1997. These effects were (1) a formation of
slow, dense, and cool heliospheric plasma layer because of the
deceleration of the flow due to the ADF active filament on the HCS
current sheet, and (2) generation of the shock wave both due to some
acceleration of the flow from the coronal hole induced by the
energy input from CME of 12 May 1997 and to lowering of the
threshold for the generation of a rapid magnetosonic shock wave
while it was propagating though the dense and cold plasma of the
active filament.

To interpret qualitatively the velocity profiles in CR 1921
and
CR 1922, it is useful to take into account two factors, one
accelerating and the other decelerating the flow. The acceleration
could be caused by AR 8026 (S24 E09) and AR 8038 (N21 W09)
(Solar-Geophysical Data, Boulder, NN 637-640,
1998)
in which there
were observed flares of the sn and 1n balls on 16 April and 12 May,
respectively. The deceleration could be due to the phenomena of
"loading" of the high-velocity flows by the active filaments'
substance (Table 1).

4.2. Dynamics of the Profiles of the Solar Wind
Proton Concentration
(Figure 4)

The variations of
N in the high-velocity flow before and after the
solar activity complex destabilization (CR 1921 and CR 1922) are
the most interesting.

These profiles are similar, but there are also considerable
differences between them. First, a strong and durable increase of
the concentration in the head part of the flow was observed during
both rotations. However, in CR 1921 it was a rather smooth compact
variation with a sharp increase and smooth decrease, almost
entirely located on HCS (between the sector and subsector
boundaries, the latter in this rotation almost completely
coinciding with the flow surface). As for the increase in CR 1922,
it was a more irregular growth with a sharp depletion of the
concentration at the front boundary of the cloud. The increase
consisted of two parts: the increase during the development phase
of the disturbance (in front of HCS and shock front) and the
increase in the shock layer in front of the magnetic cloud.

Second, the concentration within the cloud is depleted in both
rotations. However, the concentration profile in CR 1921 is almost
flat ( N const), whereas this profile in
CR 1922 is irregular
with a decrease of
N in the magnetic cloud and a train of
high-amplitude fluctuations upward along the flow from the cloud.

This feature of the
N profile in CR 1922 may be qualitatively
explained by the presence of relatively dense heliospheric plasma
sheet formed in the interaction of ADF and HCS
[Ivanov and Romashets, 1999].
The latter (as it has been shown above, see
Figure 2a)
was adjacent to the low-latitude coronal hole. The
N profile in the previous rotation CR 1921 was more smooth since
in
this rotation, though a hole in the vicinity of HCS was observed,
there was no such powerful filament as ADF (Figure 1c).

Ivanov [[2001]
proposed calling such increase of the density
within high-velocity flows a "loading" by active filaments'
substance. Moreover, the high-velocity flow in question was "loaded"
by magnetic cloud as well. It should be noted that
Webb et al. [2000a]
indicated that this cloud is located in the front part of
the high-velocity flow, though they did nor discuss the nature of
this flow; it was presumed that the flow was a posteruptive flow
being part of the halo-like CME formed in AO 8038.

Figure 5

Figure 6

Figure 7

Figure 8

4.3. Dynamics of IMF: Magnetic Cloud on Heliospheric
Current Sheet

At the stage of the activity complex development (CR 1920
and CR 1921),
the near-Earth satellites should have been located mainly in
the IMF negative sector (Figure 1). This statement is confirmed by
the IMF measurements at the Wind satellite (Figures 5, 6, 7, and 8).
Actually, the field almost all the time is directed sunward
( Bx >0 ) and westward ( By < 0 ).
Only during some short periods does the IMF
direction change to the opposite. The magnetic fluctuation level
is high, the fact additionally indicating that the satellite orbit
is close to HCS and that the latter is unstable. The
destabilization of the solar activity complex (CR 1922) initiated a
near-Earth disturbance during which the Wind satellite exited the
positive sector and (after triple crossing of the sector boundary
at 0038 UT, 0518 UT, and 0950 UT on 15 May) entered first the
magnetic cloud (0951 UT) and then, after exiting the cloud
(~2330 UT on 15 May) almost till 0300 UT of 17 May was located mainly
in the IMF negative sector ( Bx > 0,
By < 0 ). Therefore, the
magnetic cloud in the 15 May disturbance was located at the
heliospheric current sheet and its nature might have been closely
related to the magnetic field of this layer, the field of the
heliospheric electrojet
[Ivanov and Romashets, 1999].
The fact that
the cloud was close to HCS was not taken into account in the known
publications
[Watari et al., 2001;
Webb et al., 2000b]
dedicated to
modeling of this cloud by solving the inverse problem in which the
geometric characteristics of a powerless cylinder configuration are
determined from the experimental profiles of the
Bx,
By, and
Bz components.

Moreover, it was assumed in the above-mentioned publications that
the cloud was a magnetic bundle, a part of the coronal mass
ejection from AR 8038.
Below we consider in detail various properties of the
solar-interplanetary disturbance of 12-16 May 1997 including the
nature and modeling of the magnetic cloud.

5. Near-Earth Disturbances of the Interplanetary Medium
of
14-18 May 1997

The dynamics, structure, and configuration of the near-Earth
interplanetary disturbance of 14-18 May 1997 are considered in
this section in detail. The aim of the consideration is to reveal
characteristics of this disturbance which are detected
experimentally and can be used for testing various MHD models
oriented to this very disturbances.

5.1. Structure and Configuration of the Near-Earth Disturbance

The structure of this disturbance in the interplanetary medium near
the Earth was considered by
Ivanov and Petrov [1999],
Ivanov and Romashets [1999],
Ivanov et al. [1999],
and
Webb et al. [2000a].
Three phases of this disturbance were detected:
preliminary (the development phase G) (~0810 UT on
14 May-~0110 UT on 15 May), main phase M
(from 0110 UT on 15 May to ~0300 UT on 16 May),
and recovery phase R (after 0300 UT on 16 May).
It was shown that the G phase is a monotonic variations of
the IMF
components modulated by almost continuous train of nonlinear Alfvén
waves and rotational ruptures
[Ivanov and Petrov, 1999;
Ivanov et al., 1999].
It was suggested that the G phase is a preliminary
result of the interaction between ADF and HCS contained in the
activity complex in question (Figure 2a). Further on, the sporadic
coronal mass ejection CME from AR 8038 becomes involved
into the
interaction and the main phase of the disturbance occurs. Namely
during the main phase of the disturbance the shock wave and
magnetic cloud are observed. However, one should bear in mind that
the shock wave and cloud are interacting with HCS and slow dense
solar wind from the active filament ADF
[Ivanov and Romashets,1999].
Moreover, it was found (Figure 2a) that the high-velocity
flow from the low-latitude coronal hole CH also becomes involved
into interaction, so as a result we have in this disturbance a
complicated CME-CH-ADF-HCS interaction. This fact should be taken
into account in interpretation and modeling of this disturbance.

Below we consider in more detail the structure and configuration
of
the main phase of this disturbance.
Table 2
shows the results of determination of the
jN and
qN showing the direction of
the prevailing propagation of
nonlinear waves and ruptures (RDs) during the development phase of
the disturbance. Also presented in Table 2 are normals to the
heliospheric current sheet HCS1, HCS2, HCS3, and HCS4, to the
forward shock wave
Sf, and to the magnetic cloud boundary in the
entrance
RI and exit
RII points.

The scatter matrix method is used to determine these directions;
the coordinates are solar ecliptic. The normals to
Sf and
RI determined by
Berdichevsky et al. [[2000]
and calculated by us from
the magnetic cloud geometric characteristics obtained by
Webb et al. [2000a],
respectively, are marked by asterisks.

Figure 9

Figure 10

It follows from Table 2 that (1) the disturbance propagates
westward in its front part (1500 UT on 14 May-1000 UT on 15 May),
(2) the conclusion of
Webb et al. [2000a]
that the magnetic cloud
axis lies slightly below the ecliptic plane ( qN > 0 for the
normal to
RI and
qN < 0 for the normal to
RI ), is
confirmed, (3) a triple crossing of HCS (Figure 9) is observed, the
qN sign changing which makes
possible a schematic
presentation of the disturbance geometry shown in Figure 10, and
(4) after the first and third crossings of HCS, there were observed a
forward shock front
Sf and the magnetic cloud front boundary
RI,
which formed closely located pairs of the raptures HCS1-
Sf and
HCS2-
RI (Figures 9 and 10).

These results confirm the assumption that since the complicated
solar source of the disturbance included a piece of the
heliospheric current sheet( Figure 2a), this fact should have been
manifested in the structure of the near-Earth disturbance.
Certainly, an explicit MHD model should reproduce all the sequence
of the structural elements shown in Table 2.

Concluding, we note that the observed distance of the forward shock
wave from the front point of the cloud was in this disturbance
1.3 1012 cm
which (under the Mach number
M = 2.1 [Berdichevsky et al., 2000])
is very close to the theoretical value for a cloud
with a shape of a circular cylinder
[Belotserkovskiy, 1957].
However, the normals to
Sf and
RI are directed southward and
northward, respectively (Table 2), which provides difficulties in
connecting
Sf to the magnetic cloud.

5.2. Forbush Effect in the Galactic Cosmic Rays

Figure 11

Figure 11 shows variations of the density
A0 and anisotropy
Axy of the rigid (~10 GV) component of the galactic
cosmic rays
obtained by the global survey method from the neutron monitors
network. The variations consist of a gradual preincrease
(~1500-2400 UT on 14 May), more sharp fluctuations of the decrease and
increase (~0000-1000 UT on 15 May), the "main" depletion
(~1000-2400 UT on 15 May), a small smooth fluctuation of the
increase-decrease (0000-0600 UT on 16 May), and further slow
recovery.

The amplitude of the effect (~1-1.5%) is anomalously small
from the point of view of statistical relations with the
interplanetary medium characteristics
[Belov et al., 2001].
For
example, the expected values of the amplitude for the disturbance
of 15 May 1997 ( V max = 500 km s
-1,
B max = 25 nT) is

which is by a factor of 4.5 higher than the observed amplitude.

The temporal behavior of the amplitude is unusual since large-scale
Fluctuations of GCR with the amplitude comparable to that of the "main"
depletion has been observed both before and after the Forbush
decrease.

Thus the Forbush effect is anomalous by its amplitude and
unusual
by its shape. The galactic cosmic ray (GCR) variations were
compared to the MHD structure of the disturbance (Figures 9 and 10,
Table 2)
and led to the following conclusions: (1) The preincrease
was observed in the negative IMF sector from the moment of the
sharp decrease of the propagation direction of the nonlinear MHD
waves and raptures (RD) to the moment of crossing HCS1, entering the IMF
positive sector, and arrival of the shock front. (2) The sharp
fluctuations of the increase, decrease, and again increase of
A0 almost coincided with the moments of the HCS1, HCS2, and HCS3
crossings of the heliospheric current sheet, respectively, and were
observed within the positive, negative, and again positive IMF
sectors. (3) The "main" depletion was observed within the magnetic
cloud ( RI-RII ).
The nature of the Forbush effect being closely related to the MHD
structure of the near-Earth disturbance, is nevertheless still
obscure.

The preincrease has the most clear interpretation as a typical
event caused by the reflection and acceleration of particles at the
shock front
[Dorman et al., 1970;
Ruffolo, 1999;
Ruffolo et al.,1999].

For the model with an exponential decrease of
A0 upward the flow
from
Sf [Ruffolo, 1999]
one can write

where
B and
C are constants,
Z is the coordinate perpendicular to
the front,
k is a spatial scale of the preincrease (equal to
D/U in
the theory and evaluated in the experiment as
Vsft where
D,
U,
Vsf, and
t are the diffusion coefficient, the solar wind
velocity in the shock front coordinate system and along the magnetic
field, and the duration of the preincrease. In the event in
question:
Vsf =380 km s
-1,
t = 12 hours,
k-1 = 1.5 1012 cm,
D 3.5
1019 cm
2 s
-1,
and the transport path of particles
with rigidity 10 GV
l II = 3D/C 3.5 1012 cm. Moreover,
DA0
1% in this event (Figure 11)
and the ratio of the tangential
components of the magnetic field at the shock front is
Bt 2)/Bt
1 4 and agrees by a factor of 2 with the
theoretical model of the preincrease
[Ruffolo, 1999].

Still more difficult is the problem of interpretation of the GCR
variations after the shock front passage. In general, one can
conclude that these variations are due to the GCR interaction with
the "crimped" HCS and magnetic cloud. A complete answer requires a
knowledge on how the specific MHD disturbance structure has
occurred. In general, it is clear that it was formed as a result of
the interaction with the HCS of the high-velocity flow activated by
CME from AR 8038 (see
sections 4.1-4.3). However a specific
scenario of the formation of this structure still is obscure. One
of the possibilities is to assume that the entire complicated
structure of the MHD disturbance, including the "crimped" HCS and
magnetic cloud, has been formed in the interaction of the activated
high-velocity flow with HCS. This scenario is close to the one
suggested earlier by
Ivanov and Romashets [1999].
In the second
version, the "crimped" HCS (but not the magnetic cloud) and
corresponding fluctuations of GCR could have appeared in a
collision with HCS of the magnetic cloud which has come from AR 8038.
It is a modification of the
Webb et al. [2000a]
scenario in
which HCS was not taken into account.

Thus unusual shape of the Forbush effect provides no arguments in
favor of this or that hypothesis of the magnetic cloud origin.
At the same time, the properties of the Forbush variations found
above and their relation to the MHD disturbance confirm a need to
take into account the SME interaction with HCS while interpreting
and modeling this disturbance.

The anomalously small amplitude of the Forbush effect also
cannot
be a serious argument in favor or against any of the
above described hypotheses, though we think that this amplitude is
more consistent with the model of the cloud formation on HCS.
Relating to this, we emphasize that the "main" depletion of
A0 in
the magnetic cloud (Figure 11) looks in this event as a normal
decrease typical for the entire IMF positive sector. Actually, if
one takes into account the sequence of three GCR decreases in the
positive sector (the fluctuation behind
Sf, the "main" depletion
in the magnetic cloud, and the decrease after ~0400 UT on 16 May),
then the "main" depletion has an intermediate value by the
amplitude which indicate to a recovery of
A0 after the strongest
decrease up to 1.5% in the sharp fluctuation behind the shock
front.

To chose the hypothesis of the magnetic cloud origin, it
would have
been useful to consider in detail the variations of all the
components of the anisotropy vector
A1. Figure 11 shows the
amplitudes of its components in the ecliptic plane
Axy. Within the
cloud the amplitudes fall down to the minimum value at the nearest
distance from the cloud axis.

In this event a considerable component
Az 1% directed
northward was observed during the entire period of the cloud
passage. Thus one can make a preliminary conclusion that the data
on the anisotropy vector variations are in favor of the hypothesis
of the cloud formation on HCS, for example as a result of the
mechanism which is proposed to explain formation of plasmoids in
the magnetospheric tail
[Kivelson and Khurana, 1995]:
reconnection
under a tiring-instability in the neutral layer.

The Forbush effects of this disturbance were studied also in the
integral ( E > 50 MeV) GCR flux on the basis of the observations at
the SOHO satellite
[Makela et al., 1998]
and in the
ultra-relativistic part of the spectra on the basis of the data of
the muon telescope network
[Munakata et al., 2000].

5.3. Radiobursts of Type II

This event is considered as a rare radio-rich event
[Gopalswamy et al., 2000]
because of the presence in it together with the meter
and kilometer radiobursts also of the deka- and hectometer
radiobursts. According to the observations on board the Wind
satellite
(http://lep694.gsfc.nasa.gov/waves), there is no smooth
transition between these wave ranges, whereas in the interplanetary
medium the burst is weak and discontinuous
[Reiner et al., 1998],
the fact allowing us to assume that it was screened by the
heliospheric plasma layer
[Ivanov and Romashets, 2001].

The weak radioburst at the frequency
f0
150 kHz occurred in
the interplanetary medium about 1800 UT on 13 May and lasted with
some variations of the spectrum and small interruptions till ~0300 UT
on 14 May. In the interval between 0300 and 0300:30 UT a rapid
decrease of the frequency down to
f1 90 kHz was observed
with
a transition to a very narrow emission band. These variations in
the spectrum may be interpreted as a crossing by the shock front of
a sharp boundary from more dense and inhomogeneous medium to less
dense and more homogeneous medium. (If we estimate the density
change approximately as the ratio of the frequencies squared, we
obtain the density decreases by approximately a factor of 2.5.) For
example, the above fact might have been interpreted as an arrival
of the shock wave into the vicinity of the heliospheric current
layer or rarefied magnetic cloud.

6. Discussion

Below we briefly discuss the results of the papers in the following
topics: solar sources, near-Earth disturbances,
solar-interplanetary phenomena, and modeling. The accent is made
upon the particular results and problems of the study of the 12-18 May 1997
disturbance.

6.1. Solar Sources

Analyzing the complex of solar, interplanetary and near-Earth data
we show in this paper that (1) the active complex
AR(sf)-CH-ADF-HCS, i.e., a complex solar source, was a cause of the
near-Earth disturbance; and (2) this active complex was formed near the
"joint" of the sector and intersector boundaries of the model
coronal magnetic field on the source surface as a result of slow
dynamics of the photospheric regions of the open field of the Sun.

These results specify ideas on the solar source of the near-Earth
disturbance as compared both with the publications in which only
the active region AR 8038 with the
sf flare and small active
filaments in this region is taken into account
[e.g.,
Webb et al., 2000b],
and with the
Ivanov and Romashets [1999],
who did not take
the low-latitude coronal hole CH in the Fe XIV line
into account.

Certainly, in the source (especially on the Sun) the most
pronounced were the sporadic phenomena in AR 8038 (flare, Morton
wave, CME)
[Hudson et al., 1998;
Plunkett et al., 1998;
Sheeley et al., 1999;
Thompson et al., 1998;
Webb et al., 2000b];
however,
the input of these sporadic events into the near-Earth disturbance,
in our mind, needs specification. Also the input into this
disturbance of the interactions of SME with HCS, ADF, and CH should
be taken into account. This particular event confirms the tendency
(which has been formed during the recent decade) to interpret the
majority of near-Earth disturbances as complicated events with
inputs from complex solar sources
[Bravo et al.,1998;
Crooker and McAlister, 1997;
Gonzalez et al., 1996;
Ivanov, 1996].
According to the terminology suggested by
Ivanov [1998],
the 12-18 May 1997 disturbance was a
flare-hole-filament-strimmer one.

This event is the third in a series the events studied in which a
complex solar source is formed near the joint of the sector and
intersector boundaries as a result of slow dynamics of the
photospheric regions OR of the open solar field. In two other
cases, in July 2000
[Ivanov and Kharshiladze, 2002]
and January
1997
(Ivanov et al., submitted manuscript, 2002)
this fact was also emphasized. The
tendency of activity complex to appear in the interaction region of
ORs in the vicinity of sector and intersector boundaries
[Ivanov et al., 2001b],
in our opinion, agrees with the results which indicate
to a frequent appearance of CME at the intersector chains of
coronal streamers
[Eselevich,1995;
Fainshtein, 1997;
Hundhausen,1993].

The problem of the interactions in a complex source which may begin
on the Sun and be continued in the interplanetary medium till the
arrival to the Earth orbit, is very important. In this event the
source consist of closely located AR, ADF, HCS, and CH, and so one
could expect a manifestation of these interactions in the data of
solar observations.

One of such interactions (AR-HCS-CH) could, in our opinion,
be
manifested in the strong nonhomogeniety of the UV radiation front
which propagated over the entire solar disc with the Morton wave
after the flare at 0443 UT on 12 May
[Thompson et al.,
1998, Figure 2].
Thompson et al.
noted only the increase in the emission
and deceleration of the part of the front propagated northward, the
fact being interpreted as an interaction of the shock wave with the
north polar coronal hole. To complete, we compare Figure 2 of
Thompson et al. [1998]
to Figure 2a
of this paper and note
that the largest irregularity in the emission (at 0450-0507 UT in
the solar disk segment with
DF = 15o,
DL 20o and with the center at
F = S5,
L = W10 from
the flare meridian) almost completely covers the nearest to the
Earth piece of HCS and the low-latitude CH which were part of the
considered solar activity complex. This fact may be considered as
an indication to an interaction of the shock wave with HCS and CH
and to possible consequences of this interaction (besides the
nonhomogeneous front of the UV radiation, those are also a
formation of the magnetic bundle
[Gosling, 1990;
Marubashi, 1986]
and of the HCS bow
[Wu and Dryer, 1997]).
This fact has not been
taken into account in the MHD modeling of the Morton wave in the
12 May 1997 event.
Wu et al.'s [[2001]
Figure 4
shows a
comparison of the observations of the UV radiation front
[Thompson et al., 1998]
with the simulation results obtained under an
assumption that the wave propagates in a medium with constant
values of the density and temperature. As a result, the model front
was homogeneous in all directions, including southwestward from
the flare where the filament was located and a strong
nonhomogeniety of the emission was observed. Finally, modeling a
near-Earth disturbance, various alternatives should be considered
of the formation of the magnetic cloud observed
in
the near-Earth disturbance
[Ivanov and Romashets, 1999;
Makela et al., 1998;
Shodhan et al., 2000;
Watari et al., 2001;
Webb et al.,2000a, 2000b].

6.2. Near-Earth Disturbance

The near-Earth disturbances in the interplanetary medium on
14-16 May 1997 have been studied relatively weakly. The main attention
has been paid to the MHD structure of the development phase of this
disturbance
[Ivanov and Petrov, 1999;
Ivanov et al., 1999],
to
local models of the magnetic clouds based on the ideas on the
heliospheric electrojet field
[Ivanov and Romashets, 1999]
and on
powerless field of a circular cylinder
[Watari et al., 2001;
Webb et al., 2000a],
and also to the energetic particles
[Lario et al., 2000;
Makela et al., 1998;
Munakata et al., 2000;
Sanderson et al.,1998;
Shodhan et al., 2000;
Torsti et al., 1998].

It was assumed in the very beginning that this is a complicated
disturbance from the complicated solar source with the flare,
active filament and heliospheric current sheet
[Ivanov and Romashets, 1999].
Later it was found (see sections 3-4) that it is
not enough, since we are dealing with a recurrent high-velocity
flow form a low-latitude coronal hole modified by the interactions
in the AR-CH-ADF-HCS complex.

However, this fact does not exclude the question on the magnetic
cloud origin in May 1997. It was shown in section 4 that the cloud
was observed on HCS and so, besides the assumption that the cloud
is a magnetic bundle, a part of CME from AR 8038
[Gibson and Low, 2000;
Webb et al., 2000a],
it is desirable to bear in mind a
possibility of generation of this cloud on HCS in the AR-CH-ADF-HCS
complex by one of the mechanisms discussed in literature
[Gosling,1990;
Ivanov and Romashets, 1999;
Kivelson and Khurana, 1995;
Marubashi, 1986;
Wu and Dryer, 1997].

It is significant that contrary to the 5-12 January 1997
events
(Ivanov et al., submitted manuscript, 2002)
the determination of the normal to the cloud
at the point of the entrance of the Wind satellite based on the
geometric characteristics of a circular cylinder cloud in the
Webb et al. [2000a] and
Watari et al. [[2001]
calculations does not
contradict the determination of the normal by the scatter matrix
method (Table 2).
The cloud was neither strongly deformed nor
strongly compressed, so the usual method of looking for
configuration characteristics from the IMF components
[Burlaga,1988]
is applicable to this cloud. The absence of a strong "density
pulse" at the cloud rear wall and a Forbush increase of the GCR
intensity within the cloud in the May event provide additional
confirmation of the assumption on a significantly different degree
of compression of these clouds. The latter means that the May cloud
should not be classified as a "superexpanding" cloud in which
strong currents at the cloud boundary and deviations of the cloud
shape from a cylinder should be taken into account
[Cargill et al., 2000;
Schmidt and Cargill, 2001].

However, the position of the forward shock wave relative the front
boundary of the cloud does not agree completely with the
Berdichevsky et al. [[2000]
assumption that this is a deflected wave
under a quasi-stationary flowing around with the Mach number
M = 2. Actually, the normals to
Sf and
RI diverge by
30o and are
directed southward and northward, respectively. This means that
during 8.5 hours between the crossings of
Sf and
RI either the
magnetic cloud turned to the south by a jump or the wave changed
its direction sharply while crossing HCS or the cloud was not a
generator of this wave.

There is one more "configuration" problem of relation of
this cloud
to a solar source. It looks like the following.
Webb et al. [2000a]
came to the conclusion that the cloud axis lies southward from the
Earth below the ecliptic plane. Our determinations by different
methods confirm this conclusion (Table 2). To agree the position of
AR 8038 (N23 W09) as a possible source of this cloud with the
position of the cloud axis one has to suggest that on its way from
the Sun to the Earth the cloud was shifted southward almost by
30o and turned anticlockwise by 50-
60o [Webb et al., 2000a].
No particular evidences have been presented in favor of these
suggestions except the reference to the rotation in the proper
direction of the active filament from this AR. In the cloud models
oriented to its origin on HCS (for example, in the model with the
heliospheric electrojet
[Ivanov and Romashets,1999])
the problem
is easily eliminated since in the cloud observation, moment HCS in
this event was located southward from the Earth (see section 4).

If we discuss the hypothesis on the shift and turn of the cloud, we
should pay attention to the following: (1) the CME-halo was more
bright at the north and east than at the south and west
[Plunkett et al., 1998];
(2) some role was played by the interaction of the
coronal shock wave with the HCS and CH located south-westward (this
fact is manifested by the strong nonhomogeniety of the Morton wave
[Thompson et al., 1998,
Figure 2];
this interaction, in
particular, could have generated the reflected wave pushing the
cloud from AR 8038 northeast; (3) one has to agree the spheromack
model by
Gibson and Low [[2000]
who applied
it to AR 8038
AR in which the magnetic bundle (cloud) in the initial position was
located along the neutral line of AR, whereas in the interpretation
of
Webb et al. [2000a]
the initial position of the cloud was
perpendicular to this line.

What was happening with this disturbance on the way from the Sun to
the Earth one can only assume on the basis of the Forbush effects
observed at the network of neutron monitors (see
section 5.2)
and muon telescopes
[Munakata et al., 2000]
and also on board the
SOHO satellite
[Makela et al., 1998].
These were observations of
the particles with the rigidity of about 10 GV, 16-890 GV, and the
integral flux of the particles with the rigidity above 50 MeV,
respectively. It would be interesting to compare these results to
the measurements in the spectrum of the kilometer emission of type
II (see
section 5.3).

The first thing we would like to point out to is an astonishing
coincidence of the moments of the integral GCR flux preincrease
observed on board SOHO
[Makela et al., 1998]
with the moment of the
appearance of a weak kilometer radioburst of type II (see
section 5.3). This coincidence occurred at about 1800 UT on 13 May
and may be interpreted as a very quick reaction of GCR at the
distance of ~0.5 AU from the Sun. Further on, till the arrival of
the shock front to the Earth, the particle density increased by
about 4%. Contrary to the Forbush effects in the ultrarelativistic
part of the spectrum, the density variations in the integral flux
behaved in agreement with the statistically mean characteristics
[Belov et al., 2001]:
a two-step (at the shock wave and in the
magnetic cloud) decrease was observed, and the expected value of the
decrease amplitude (~7%) was reached.

It is worth also noting that the beginning of the rigid GCR
preincrease (~1500 UT on 14 May, see Figure 11) was close to the
time of the sharp change of the propagation direction of the
nonlinear MHD oscillations (RDs in Table 2). Not long before that
(0300-0400 UT on 14 May) a sharp reconstruction of the spectrum of
the
type II
radioburst
occurred according to the observations on
board the Wind satellite (see
section 5.3). It is possible that
these events indicate the fact that
shortly before its arrival to the Earth
the interplanetary wave
took part in the
interaction with large-scale irregularities of the interplanetary
medium (magnetic cloud, HCS); however, the exact physical sense of
these interactions is still obscure.

An explanation of the cloud spirality may present a difficulty in
the hypothesis of the cloud formation on HCS. In this case
(according to the spirality rules for the solar cycle
[Bothmer and Schwenn,1998])
it was a left-spiral SEN cloud
[Webb et al.,2000a].
The cloud spirality near the Earth is easily explained by
the magnetic field spirality of the bipolar group in AO 8038
[Webb et al., 2000a];
however, it is not clear which should be the
spirality of the cloud generated on HCS. In relation to this we
note that the same problem with the spirality appears in
the work by
Mulligan et al. [[2001],
who have found a cyclic variation in the
positive correlation between the directions of the magnetic clouds
axes and the HCS inclination to the equator plane. In the event in
question, the magnetic cloud could have been formed at the part of
HCS formed by remote interactions of the pair of open regions
+3/-1 (Figure 2a),
which have the same longitudinal shift as the sunspots
of the bipolar group in AR 8038. Probably, solving this problem,
one would be able to explain the spirality of the magnetic cloud on
15 May 1997 without denying the assumption on its generation on
HCS.

6.3. Scenario and Model of the Disturbance

This event is presented above as consisting of a slow phase of
gradual formation of the activity complex and rapid phase of this
complex destabilization with a strong disturbance in the space
between the Sun and the Earth.

Obviously, currently there is not enough either experimental data
nor theoretical concepts which would make it possible to describe
and model the entire chain of the events unambiguously and in
detail. The most problematic
aspect remains
the description and
explanation of the events during the slow phase. It is suggested
[Ivanov et al., 2001b]
that the photospheric regions of the modelled
magnetic field of the Sun together with large-scale background
magnetic fields
[Bumba and Howard, 1965]
are a result and
manifestation of giant modes of the convective instability
[Fox et al., 1998;
Simon and Weis, 1968;
Wilson, 1987, 1992;
Wilson et al.,1990;
Yoshimaru, 1971],
the interactions between them generating
solar activity complexes. This view on the origin and role of the
open magnetic field of the Sun is an alternate hypothesis on
generation of the open field in the active solar regions
[Leigton,1964;
Wang et al., 2000].
Not rasing a discussion on this
fundamental problem, we would like to draw attention to the fact
that (as shown in
sections 3.2 and 3.3) the appearance
of the relatively separated open regions OR
-1,
+2, and
+3 (Figures 1
and 2) and their tendency to a convergence preceded by almost one
rotation the appearance of AR 8026 and 8038 and formation of the
activity complex.

One more perspective approach to the interpretation of the dynamics
of the photospheric regions of the open magnetic field of the Sun
could be based on the concept of the Rossbi waves generated in the
convective zone of the Sun and having the characteristic linear
dimensions close to those of OR ( 0.5 R )
and a similar life-time
(about 10 solar rotations)
[Gilman,1969;
Tikhomolov and Mordvinov,1997].

The rapid phase scenario has more reliable and various experimental
backgrounds than the slow phase scenario and makes some elements of
modeling possible. It is mainly true for the events near the Sun
and Earth. The data and ideas on what has been happening in the
outer corona and interplanetary state still are limited and
controversial.

In the scenario of the rapid phase which started on 12 May 1997
with sporadic events in AR 8038
[Gopalswamy et al., 2000;
Hudson et al., 1998;
Plunkett et al., 1998;
Thompson et al., 1998;
Webb et al., 2000a, 2000b],
it is assumed (see sections 3 and 4) that in the
vicinity of AR 8038 AR and the helioprojection of the Earth, there
were located an active filament, part of the heliospheric current,
and low-latitude coronal hole, all of them forming a united
activity complex (Figure 2a).

Because of that, the sporadic disturbances formed in AR 8038 should
have interacted with the above indicated structural formations and
the results of this interactions could have been manifested: in the
strong inhomogeneity of the Morton wave front (see
section 6.1)
in the vicinity of the photosphere; in decameter and hectometer
radioemission in the outer corona; in the sharp changes of the
spectrum of the kilometer radioburst of type II (see
section 5.3); in multiple crossings of HCS associated with the forward
shock wave and magnetic cloud in the near-Earth environment; and in
the anomalous Forbush effect in the MHD and GCR precursors of the
disturbance (see
sections 5.1-5.2).

Certainly, the observations by
Gopalswamy et al. [[2000],
Hudson et al. [1998],
Lara et al. [[2000],
Plunkett et al. [1998],
Sheeley et al. [1999],
Thompson et al. [1998],
and
Webb et al. [2000a, 2000b]
quite definitely show that AR 8038 was a source of the coronal mass
ejection with the shock wave, magnetic cloud, ejection of a small
filament, and posteruptive flow from the transient coronal holes.
So one can assume that near the Earth was observed the same CME
which had started from AR, if on the way to the Earth the cloud had
shifted southward by almost
30o and turned around the
longitudinal axis by
45o [Webb et al., 2000a].

The above mentioned condition is, in our opinion, the weakest point
of this scenario. Moreover,
neither the
complicated
complex of solar sources
indicated above
nor possible interactions and their
consequences, especially in the near-Earth space, are taken into
account in this scenario. Nevertheless, the scenario worth further
study
due to
both
the direct relation to the observed properties
of AR 8038 (the comparison of magnetic clouds in AR and in the
magnetic cloud, cloud spirality according to the Bothmer and Rusta
rule) and the fact that the interactions and detailed features
of the near-Earth disturbance found in this paper partially are
able to be agreed with the
Webb et al. [2000a]
scenario. For
example, if the consequences of the HCS collision with the magnetic
cloud from AR were observed near the Earth, then one can consider
as a result of this collision the multiple crossings of HCS in
front of the front cloud boundary accompanied by large-amplitude
fluctuations of the MHD parameters and GCR intensity (Figures 3, 4, 5, 6, 7, 8, 9, 10, and 11).

A different scenario of these disturbances is known in which the
complexity of the source was taken into account, possible
interactions were discussed, and it was suggested that the magnetic
cloud was originated on HCS and may be presented as a heliospheric
electrojet field
[Ivanov and Romashets, 1999].

Taking into account the results of this study, this scenario may be
presented in the following way.

1. On 12 May CME (the forward wave, magnetic cloud, posteruptive
flow) starts from AR 8038, the south-western flank immediately
interacting with ADF, HCS, and CH, this fact being manifested in
the strong inhomogeneity of the Morton wave front [Thompson et al., 1998]
and occurrence of the
deca- and hectometer radiobursts of type II
[Gopalswamy et al., 2000].

2. A new magnetic cloud (an analog of a plasmoid in the
magnetotail
[Kivelson and Khurana, 1995])
is born near the Sun in
the CME-HCS interaction act and is captured into the high-velocity
flow from the low-latitude coronal hole CH loaded by the substance
of the active filament ADF.

3. The forward shock wave attenuates
strongly, a return wave is formed, the magnetic cloud is pushed out
of AR northeastward. This fact is manifested by the almost
complete disappearance of the II-type radiobursts
[Reiner et al.,1998]
(see
section 5.3) and a high intensity of the halo in its
northeastern part
[Plunkett et al., 1998].

4. The posteruptive
flow from AR overtake the slower "loaded" high-velocity flow from
CH and accelerates it with formation of a shock wave in the region
of the interaction of these flows. These facts are confirmed by the
occurrence of the kilometer radioburst of type II at about 1800 UT
on 13 May and correspond to the
Gopalswamy et al. [1998]
hypothesis
on generation of kilometer radiobursts in two-flow interaction
acts.

5. At ~0300 UT on 14 May the newly generated wave reaches the
rear wall of the magnetic cloud and enters it (that is confirmed by
the sharp change of the spectrum of the radioburst of type II, see
section 5.3), compresses and accelerates the cloud.

6. Subsequently, the acceleration of the cloud associated
with HCS
leads to oscillations ("crimping") of HCS (Figure 10) with a
formation of a magnetic "mirror" responsible for the preincrease of
GCR (Figure 11)
and GCR fluctuations in front of the front boundary
of the cloud.

The problem of the near-Earth cloud origin still stays under
discussion; however, it seems necessary to take into account the
complicated source and interactions in a qualitative scenario of
the 12-16 May 1997 disturbances to formulate a set of experimental
limitations to the results of MHD modeling of such simple (at the
first sight) events of solar-terrestrial physics.

7. Conclusion

Applied to the solar-interplanetary disturbance of 12-18 May 1997,
a possibility is demonstrated to study complicated solar sources,
structure, configuration, and dynamics of near-Earth disturbances
using slow dynamics of the photospheric regions of the open lines
of the magnetic field of the Sun and a broad complex of solar
observations and measurements in the interplanetary medium.

Is shown that (1) the compact activity complex formed in April-May
1997 in the vicinity of the "joint" between the sector and
intersector boundaries of the magnetic field on the source surface
was a cause of the disturbance in question; (2) the complex included
also an active region, active filament outside this region, and
coronal hole; (3) the complex generated in the interplanetary medium
a high-velocity magnetoplasma flow with the life-time of about 4
solar rotations (from the birth to the decay);
(4) an interplanetary
disturbance with the three-phase temporal dynamics typical for the
disturbances in the vicinity of the heliospheric current sheet was
observed in April-May 1997 near the Earth; (5) the destabilization of
the 12 May 1997 complex led to a significant modification of the
near-Earth disturbance: to the appearance of a forward shock wave
and magnetic cloud, to the multiple crossings of the heliospheric
current sheet and to the flow acceleration; and (6) the close relation
of the magnetic cloud to the heliospheric current layer, its
geometric characteristics, and anomalously low amplitude of the
Forbush decrease make possible an assumption that the magnetic
cloud have been formed as a result of a reconnection of the
magnetic field in the neutral layer of the heliospheric current
layer.

Acknowledgments

The authors are grateful to R. Lepping, K. Ogilvie,
and the CDA Web group for the data of IMF and plasma
measurements on board the Wind satellite, V. Kaiser for the data on
the radiobursts of type II obtained in the Wind/Waves experiment,
T. Hoeksema for the data of the measurements of the photospheric
magnetic field on board WSO, and A. I. Zavoykina for her help in
preparation of the paper. This work was supported by EU/INTAS-ESA
(project 99-00-727) and the Russian Federation Program "Astronomy".